I think technically, you are correct (the best kind of correct), we can never 100% know unless they tell us... But as umpires the rules require us to know. Anytime you make a ruling based on intent you could be wrong, but that's part of the job. Umpires are asked to do the impossible (be perfect) a lot. One of the reasons it's impossible is for precisely the reason you've pointed out.
On this play, intent to interfere with the play actually happening is required. Running inside isn't interfering with any play until there is an actual play being made. So, until the fielder throws the ball (or, as a CYA for my statement, perhaps gets ready to throw the ball) there is no play to interfere with. If, after the throw has been made, the runner who knows where the ball is deviates to get in the way, that's interference. But if the deviation happens before, there is nothing to interfere with.